NOx and other air pollutants, such as CO and VOCs, are often found in combustion flue gasses. Selective catalytic reduction (SCR) is often used to control NOx. In particular, cheaper and more effective technology is desired to control NOx emissions from large combustion sources (power plants, large boilers, etc.).
There has been research over the past decade on using hydrogen peroxide to oxidize NOx in flue gas to more scrubbable species. The hydrogen peroxide is injected into hot flue gas and thermally splits to form very active free radicals that attack the NOx so that downstream scrubbing can remove the species created. Unfortunately, this activation of hydrogen peroxide does not readily occur at low temperatures by itself. Thus, there is a desire for technology that may also be effective even at relatively low-temperatures.
Attempts have been made to remove air pollutants from flue gasses, and, in some cases, at relatively low temperatures. For example, U.S. Pat. No. 5,670,122 to Zamansky et al. discloses a method for removing air pollutants from combustion flue gases. The method comprises adding hydrogen peroxide and/or methanol to a combustion flue gas that is between 377° C. and 827° C. The hydrogen peroxide and/or methanol react with the air pollutants in the flue gas and remove nitric oxide, sulfur trioxide, light hydrocarbons, carbon monoxide, and trace amounts of mercury from the combustion flue gas.
Other technologies use ultraviolet light to dissociate hydrogen peroxide for greater performance. For example, one approach places the UV light in the gas flow tube which carries the flue gas. Alternately, the UV light is delivered by a UV lamp in a large (dilute) liquid container within a separate reactor vessel. Prior research and practice used ordinary injection nozzles for the hydrogen peroxide that were separate from the UV light source.
Attempts have been made to improve the efficiency of removing air pollutants when using hydrogen peroxide and UV light. For example, U.S. Pat. No. 5,256,379 to Deloach discloses an apparatus for removing hydrocarbon contaminants from an air stream that flows through a chamber. UV light passes through the chamber and irradiates the hydrogen peroxide. The UV light source is located outside the chamber that the hydrogen peroxide passes through. The effect is that the hydrogen peroxide does not efficiently absorb all of the UV light because not all of the UV light passes through the chamber.
Moreover, prior technology is often limited because there are competing reactions involving the UV light, it is difficult to keep the UV lamps cooled, and there is a possibility of UV leakage out of the reactor. Thus, there is a need for technology that more efficiently irradiates hydrogen peroxide to be subsequently used for removing air pollutants from gases, such as flue gasses from stationary sources.